1. Field of the Invention
The present invention relates to an image forming apparatus, an image correcting method, and a computer-readable storage medium.
2. Description of the Related Art
As image forming apparatuses such as a printer (a printing apparatus), a facsimile, a copying apparatus, and a complex machine of these apparatuses, for example, there is an image forming apparatus of a so-called inkjet system that performs, using a device including a recording head (hereinafter also referred to as “head”) including a liquid ejection head for ejecting droplets of recording liquid (hereinafter also referred to as “ink”), image formation (recording, printing, and imaging are also used as synonyms) by depositing, while conveying a recording medium (hereinafter also referred to as “sheet”, however, this does not limit a material; recording medium, medium, transfer material, recording paper, and the like are used as synonyms), the recording liquid serving as liquid on the sheet.
The image forming apparatus of the inkjet system has advantages that, for example, high-speed recording is possible, recording can be performed on so-called plain paper without requiring special fixing processing, and operation sound during the recording is extremely small. Therefore, the image forming apparatus attracts attention as an image forming apparatus for offices. Various types are proposed and put to practical use in the past.
Image formation by the inkjet system is realized by applying, using a recording head in which an ink liquid chamber and nozzles communicating with the ink liquid chamber are formed, pressure to ink in the ink liquid chamber according to image information to thereby eject ink droplets from the nozzles and deposit the ink droplets on a recording medium such as paper or a film. Because the image formation is performed in a non-contact manner, there is a characteristic that recording can be performed on various recording media.
As a problem of the image forming apparatus of the inkjet system, a problem concerning unevenness of print (hereinafter also referred to as “print unevenness”, “color unevenness”, and “unevenness”) is known. Various causes are known as a cause of the print unevenness. In particular, fluctuation in ink ejection characteristics of an inkjet head is known as a problem.
The inkjet head includes a plurality of nozzles and applies pressure to a liquid chamber communicating with the nozzles to eject ink. However, it is inevitable that slight fluctuation could occur concerning the performances of the respective nozzles. Therefore, ejection characteristics of the ink are not always the same. The “ejection characteristics (also simply referred to as “characteristics”) mean the size, the speed, the arriving position, and the shape of ink droplets. Because a way of covering of the ink on a recording sheet changes according to the ejection characteristics, print unevenness is caused.
In recent years, image forming apparatuses are requested to be increased in speed and improved in image quality. To satisfy this request, the number of nozzles per one head an increase in the length of the head and nozzle formation density) tends to further increase. If the number of nozzles and the nozzle formation density increase, the number of defective nozzles also increases according to the increase in the number of nozzles and the nozzle formation density. Therefore, the problem of the fluctuation in the ejection characteristics is more important.
Concerning the problem, for example, Japanese Patent Application Laid-open No. 2006-224419 discloses a printing apparatus that corrects an input and an output for each of nozzles (corrects the number of dots to be ejected) to thereby correct characteristics of a head for the purpose of reducing banding phenomenon due to density unevenness (a phenomenon in which a dot arriving position in a connecting portion of scanning deviates and a streak-like image failure occurs because of various factors such as a sheet feeding error and backlash of the head).
However, as explained above, the number of nozzles of the inkjet head tends to increase according to the increase in the length and the density of the head. An image forming apparatus mounted with a large number of heads for improvement of a color gamut, an increase in speed, improvement of resolution, and the like is also developed. Therefore, an enormous number of nozzles have to be managed.
Further, for example, near solid, because a paper surface is almost filled with ink from the beginning, a tint less easily changes even if a dot diameter slightly changes. However, in highlight to middle (the middle of sold and highlight), because the paper surface is not fully filled with the ink, the ting easily changes. Specifically, even if nozzles are the same, in some case, a correction coefficient (a correction parameter or a correction amount of a color) is different depending on a gradation and correction cannot be performed with a uniform coefficient. In particular, in a multi-value inkjet printer that handles a plurality of droplet types (large droplets, medium droplets, small droplets, etc.), when characteristics are different depending on a droplet type, in some case, even if the medium droplets are the same, the size of the large droplets is different. Therefore, this problem becomes conspicuous.
When all the above problems are taken into account, it is necessary to prepare correction parameters by a number calculated from “the number of heads (when an image forming apparatus includes a plurality of heads)×the number of nozzles×the number of gradations” and apply the correction parameters. For example, if an image printing result is measured to create the correction parameters, it is necessary to output and measure images equivalent to “the number of heads×the number of nozzles×the number of gradations” and create the correction parameters. If such control is performed, a configuration for acquiring ejection characteristics of all nozzles and a configuration for storing and applying a large number of parameters are necessary. This leads to an increase in cost of a product, a decrease in processing speed, and the like.
To solve this problem, it is conceivable to expand a unit for correction (a correction unit), i.e., grasp a plurality of nozzles as one unit (segment), collectively apply correction parameters for each nozzle segment, and perform correction.
This makes it possible to substantially reduce measurement points for parameter generation and the number of correction parameters. However, if the correction unit is simply expanded, a correction effect decreases. For example, it is likely that a gap (a difference) in a color change occurs in a boundary of correction segments and a change of a print pattern is conspicuous. As a result, print unevenness cannot be sufficiently solved.
Therefore, there is a need for an image forming apparatus, an image correcting method, and a computer-readable storage medium that can obtain a satisfactory correction effect by dividing, in correction processing for dividing an area to be corrected into several correction segments and performing correction for a reduction in print unevenness, the area to obtain an optimum color unevenness correction effect.